Bar mills of conventional design mostly consist of two multi-grooved rolls having bearing journals at each end. The bearing chocks each are held in mill housings, which are large and expensive steel castings with an elaborate screw-down mechanism for roll gap adjustment.
In order to work a rolled bar from two sides, horizontal and vertical stands are normally alternated. Both horizontal and vertical stands have to be shifted from pass to pass in accordance with rolling program changes or roll wear. In particular, the vertical roll mill stands are, therefore, of large height, especially if the rolls are overhead driven. Besides being an expensive design, they also need high and expensive buildings. In order to perform roll changes within reasonable time, the rolls are contained in cartridges or inner housings which are located in the outer housings, again an expensive set-up.
To avoid such high costs and complications, cantilevered roll mill stands have been used to some extent. With this kind of mill stand, narrow roll disks are used on overhung mounted drive shafts. These disks contain only one or sometimes two passes, which need more changing than multi-grooved rolls; however, they are more readily accessible and more easily changed.
Cantilevered roll stands of conventional design have serious limitations because the unusually heavy rolling load has to be supported in basically one bearing, the one closest to the roll disks. The bearing at the far end of the drive shaft does not contribute very much to the load distribution.
Due to rolling technology, the working rolls are limited in their maximum diameter with regard to the cross section of the bar being rolled. In order for the material to roll more into elongation instead of spread, small diameter rolls are preferred. This, on the other hand, is detrimental to the bearing capacity which demands large size bearings. In other words, the roll shafts cannot move close enough together with minimum roll diameters because of the needed large size bearings. This is the main reason why overhung type mill stands have mostly been used in the finishing and occasionally the intermediate trains of continuous bar mills where the loading is relatively light. Roughing and breakdown stands of heavy design are not being built in cantilevered fashion because of above limitations.